1. Field of the Invention
The present invention relates to a core manufacturing device and a core manufacturing method.
2. Description of Related Art
There have been known Interior Permanent Magnet (IPM) motors in which permanent magnets for field excitation are embedded inside a rotor. Japanese Patent No. 4726105 describes an example of a manufacturing method for a rotor core for use in the IPM motors. In the manufacturing method described in Japanese Patent No. 4726105, first, a cylindrical rotor core in which a plurality of magnet insertion holes are formed is shaped, and thereafter magnet raw materials before being magnetized are injected into the magnet insertion holes through injection molding. After that, a magnetization device is disposed so as to cover the outer periphery of the rotor core, and magnetic flux is supplied from the magnetization device to the rotor core to magnetize the magnet raw materials disposed in the magnet insertion holes. Consequently, the magnet raw materials are magnetized to be turned into permanent magnets, thus completing the manufacture of a rotor core with embedded permanent magnets.
Permanent magnets (e.g. neodymium magnets (Nd—Fe—B magnets) used for field excitation for motors have such properties that the permanent magnets can be magnetized by even a weak external magnetic field as the temperature of the permanent magnets becomes higher. Therefore, it is advantageous to heat the rotor core to a high temperature in the magnetization process.
In the case where the magnetization process is performed with the rotor core maintained at a high temperature, however, the permanent magnets of the rotor core may be irreversibly demagnetized depending on the timing to detach the rotor core from the magnetization device.
The magnetization properties of the permanent magnets such as neodymium magnets are varied in accordance with the temperature as illustrated in FIG. 20, for example. In FIG. 20, C1, C2, and C3 indicate an initial magnetization curve, a B-H curve at a normal temperature, and a B-H curve at a high temperature, respectively, for the permanent magnets such as neodymium magnets. As is clear from comparison between the normal-temperature B-H curve C2 and the high-temperature B-H curve C3 of FIG. 20, the permanent magnets such as neodymium magnets have such properties that as the temperature rises, the absolute value of the coercive force becomes smaller, and the absolute value of the magnetic field corresponding to the inflection point becomes smaller. Therefore, there is the following difference in magnetic flux density of the permanent magnets after the completion of the manufacture of the rotor core between a case where the rotor core is detached from the magnetization device after the rotor core is cooled in the magnetization device and a case where the rotor core is detached from the magnetization device with the rotor core still at a high temperature after the completion of the magnetization process.
When the rotor core is attached to the magnetization device, the magnetic flux density of the magnet raw materials of the rotor core is increased from zero along the initial magnetization curve C1 by a magnetic field generated by the magnetization device. In this course, the magnet raw materials are magnetized to be turned into permanent magnets. When the magnetization of the permanent magnets is substantially saturated so that the permanent magnets are completely magnetized, the magnetic flux density of the permanent magnets reaches a magnetic flux density Bs1. After that, in the case where the rotor core is temporarily cooled in the magnetization device, the B-H curve for the permanent magnets transitions from the high-temperature B-H curve C3 to the normal-temperature B-H curve C2 as indicated by the arrow a1 in the drawing. That is, the magnetic flux density of the permanent magnets is increased from the magnetic flux density Bs1 corresponding to the high-temperature B-H curve C3 to a magnetic flux density Bs2 corresponding to the normal-temperature B-H curve C2. After that, when the rotor core that has been completely cooled is detached from the magnetization device, the magnetic field applied from the magnetization device to the permanent magnets disappears. Therefore, the magnetic flux density of the permanent magnets is varied to a magnetic flux density Bd1 on an operation point P1, which is an intersection point between the normal-temperature B-H curve C2 and a permeance line L1, as indicated by the arrow a2 in the drawing. Because the operation point P1 is positioned on a straight portion of the normal-temperature B-H curve C2, the permanent magnets are not irreversibly demagnetized.
In the case where the rotor core that has been subjected to the magnetization process is detached from the magnetization device without being cooled in the magnetization device, in contrast, the rotor core is detached from the magnetization device with the rotor core still at a high temperature. The magnetic flux density of the permanent magnets is varied along the high-temperature B-H curve C3 as indicated by the arrow b in the drawing. That is, the magnetic flux density of the permanent magnets is varied to a magnetic flux density Bd2 on an operation point P2, which is an intersection point between the high-temperature B-H curve C3 and the permeance line L1. In this event, the operation point P2 is lower than an inflection point Pcn of the high-temperature B-H curve C3. In this case, when the permanent magnets are cooled to a normal temperature, the operation point of the permanent magnets is only varied from P2 to P3. That is, compared to a case where the rotor core is cooled in the magnetization device, the magnetic flux density of the permanent magnets is demagnetized by a difference ΔBd between the magnetic flux density Bd1 corresponding to the operation point P1 and a magnetic flux density Bd3 corresponding to the operation point P3. If the permanent magnets are irreversibly demagnetized in this way, the amount of effective magnetic flux that interlinks with a stator coil may be decreased to reduce motor output torque.
In order to avoid such irreversible demagnetization, it is advantageous to cool the permanent magnets with the rotor core mounted to the magnetization device. In the case where such a method is used, however, the magnetization device cannot be used while the permanent magnets are being cooled, thus significantly increasing the cycle time of the magnetization device. This results in deteriorated productivity.
Such an issue is not peculiar to the manufacture of a rotor core with embedded permanent magnets, but also involves the manufacture of a suitable core provided with permanent magnets such as a stator core.